If you're using Google Chrome, you can install this as an application. Download Video Pinner from the Google Chrome webstore!

Pins the video on a YouTube video page to the top while you scroll so you can rage at comments while you continue to watch the video.

Tested in Chrome 30.0.1599.69 on OSX, Oct. 11, 2013

You can watch this video if you don't understand bookmarklets. It will tell you exactly how to use this.

GitHub doesn't allow you to link to a JavaScript function, so here's the code for the link. Put that link somewhere, and then drag it to your bookmarks bar.

<a class='bookmarklet' href="javascript:(function(){document.body.appendChild(document.createElement('script')).src='//raw.github.com/tforbus/youtube-fixed-video-bookmarklet/master/script.js';})();">YouTube Fixed Video Bookmarklet</a>

If you don't want to do that, you can go to this website which already contains the link, so you just need to drag and drop.

Once that's in your bookmarks bar, just click it when you're on a YouTube video page, and the code is activated.